1. Field of the Invention
The sulfur reactant container in sodium/sulfur cells is subject to attack in contact with molten sodium polysulifde and sulfur reactants. Aluminum has been identified as a nondegrading material in this environment, because it forms a continuous layer of aluminum sulfide over its exposed surfaces. Although this layer is protective, it is electrically insulating and prevents the use of untreated aluminum as an electrode material in sodium/sulfur cells.
Other metals, such as nickel, iron and their alloys, are inadequate as electrode and container materials in their untreated form. They form porous metal sulfide scales which lead to extensive physical degradation and contamination of the catholyte melt under cycling. This interferes with efficient cell operation and causes discharge capacity losses, cell resistance increases and degradation of the electrolyte.
A preferred sulfur reactant container or other electrically conducting component subject to corrosive attack by the battery reactant would be a low cost material or composite material, which exhibits the chemical stability of aluminum and the strength and electrical properties of iron or nickel or their alloys. It would therefore be desirable to convert an iron or nickel based container or component surface, such that on exposure to the cell reactants, particularly the sulfur-sodium polysulfide melt, a doped, electrically conductive aluminum sulfide scale will form, instead of the pure electrically insulating aluminum sulfide scale found on pure aluminum under the same environment.
2. Description of the Prior Art
Smialek, "Exploratory Study of Oxidation-Resistant Aluminized Slurry Coatings for IN-100 and WI-52 Superalloys" NASA'TN'D'6321 (1971), and Llewelyn, "Protection of Nickel-Base Alloys Against Sulfur Corrosion by Pack Aluminizing," Hot Corrosion Problems Associated with Gas Turbines, ASTM-STP-21 (September 1967) Grisaffe, S.J., "Coatings and Protections", Chapter 12, The Superalloys Edited by C. T. Sims and W. C. Hagel, John Wiley & Sons, N.Y. 1972 all teach the preparation of metal aluminide coatings and their application as protective coatings for superalloys in high temperature turbines.